Explanation:
As far as the eye could see,
it was a dark night at
Las Campanas Observatory in the southern
Atacama desert of Chile.
But near local midnight on April 11, this mosaic of 3 minute long
exposures revealed a green, unusually intense,
atmospheric
airglow stretching over thin clouds.
Unlike aurorae powered by collisions with energetic charged particles
and seen at high latitudes, the airglow is
due
to chemiluminescence,
the production of light in a chemical reaction, and
found around the globe.
The chemical energy is provided by the Sun's extreme ultraviolet
radiation.
Like aurorae, the greenish hue of this airglow does originate
at altitudes of 100 kilometers or so dominated by
emission from excited oxygen atoms.
The gegenschein, sunlight reflected by dust
along the solar system's ecliptic plane was still visible on
that night, a faint bluish cloud just right of picture center.
At the far right, the Milky Way seems to rise from the mountain
top perch of the
Magellan telescopes.
Left are the OGLE project and
du Pont
telescope domes.